Looking at the long, long, long term, the cosmos isn't shaping up well for sentient beings or any other living things. Hundreds of billions of years from now, the universe will start to run short of radiant energy--better known as heat--one of the necessities for life. However, two theoretical physicists have concluded there will be a small consolation: In the race to nothingness, matter will avoid finishing first.
Along with matter, life requires energy, and the ratio of the two depends, ultimately, on the fate of the universe. Until the late 1990s, scientists assumed the universe would follow one of three courses. It could continue to expand forever at perhaps a slightly decreasing rate. Or, it might reach a point of eternal balance and sit eternally between expansion and collapse. Or, it might stop expanding and begin folding back in on itself, heading toward a cataclysmic event called the Big Crunch. In all of these scenarios, protons, neutrons, and other particles will probably decay slowly into radiation, meaning the universe eventually should contain more radiant energy than matter.
Then researchers discovered that the expansion is accelerating (ScienceNOW, 13 February 2006). Galaxies are racing apart from one another at ever-increasing speeds, and at some extremely distant time, no star system will be within sight of any other. Scientists infer that a mysterious and so-far-undetected force called dark energy is propelling the acceleration.
It turns out that dark energy presents implications for the amount of radiant energy available to sustain life. Physicists Lawrence Krauss of Case Western Reserve University in Cleveland, Ohio, and Robert Scherrer of Vanderbilt University in Nashville, Tennessee, used mathematical models to simulate proton decay in an accelerating universe driven by dark energy. In the 25 April issue of Physical Review D, they conclude that beginning about 100 billion years from now, radiation will disappear from the universe faster than it can be produced by matter decay, leaving a larger and larger proportion of matter.
The acceleration is the key. Hundreds of billions of years from now, dark energy will push the universe beyond the speed of light, thereby stretching out energy waves to the point of insignificance. All heat, light, and radio waves will effectively stop interacting with matter. As a result, everything in the universe will grow inexorably colder. "In such a universe, all other galaxies and sources of energy outside of our local group of galaxies will quickly disappear" from sight, Krauss says. As a result, supplies of energy necessary for sustaining civilizations eventually will dwindle. "It is among the worst of all possible futures for life," he says.
Martin Rees of Cambridge University, United Kingdom, agrees with Krauss and Scherrer's calculations and says they have created "a straightforward scenario" for the long-term future. But he says that other than the lack of a galaxy-filled sky for astronomers to observe, locally conditions wouldn't change dramatically. Life could persist even with a steadily declining energy supply, he says, although the creatures "would be as different from humans as we are from the most primitive bugs."